Hi aron82e,
I think you are choosing between some great choices that use similar materials although there are certainly differences in the designs. Both SleepEz and mattresses.net are members of this site which means that I believe they are among the best quality/value in the country … and the extra 5% discount for your membership here doesn’t hurt either 
When you are choosing between good and good … then personal preferences and which one best matches your personal “value equation” both for the mattress itself and the options or benefits available from each merchant both before and after the sale. There is no “better or worse” in other words … only differences and preferences that may be more or less important to different people.
SleepEz has more options in terms of layering and ability to customize.
Mattresses.net is less expensive.
Other differences may include differences between their exchange or return options. In essence it will boil down to tradeoffs between all the objective, subjective, and intangible tradeoffs that are involved in any mattress purchase.
Flobeds is also great quality but for some people may not be in quite in the same value range as either of the other two for their single zoned mattresses so they may need need to be a compelling or specific reason that wasn’t offered by the other two in order to go in that direction (which could include their convoluted comfort layer or the differences in their exchange policy). Of course their zoning options for their vZone may be a compelling reason and if there was a difference that I considered important and justified the higher price or gave you more confidence that the mattress would be a better “fir” then I wouldn’t hesitate to go in this direction either in spite of the higher prices and they would also make a very good quality/value choice.
On to your specific questions …
You can read a bit more about the differences between thicker and thinner mattresses in post #14 here. If you also go here you will see someone that was heavier than you (and also preferred firmer mattresses) where SleepEz suggested a thinner model because in their case it would be fine (although others of a similar weight may choose or strongly prefer the thicker 13000). This is just to say that the overall thickness can depend on many things and there is no “rule” that would apply to all people so I would go with the recommendation of the manufacturers you are working with for their specific mattresses (which may be different from each other because of the differences in their design).
I haven’t seen this become an issue because latex is quite “sticky” and the layers tend not to shift over each other but if this were to happen over time it would be easy to open the cover and “wave” the layers back into place. Everything comes with pros and cons and the single core layer is a less complex design and the odds of shifting “in theory” would be lower but it also has fewer options to adjust the feel of the mattress and either exchange or re-arrange layers.
Again … when you are choosing among the best values in the country … the objective, subjective, and intangible differences differences between each that are most important to you and your own personal preferences would be the way to choose and I would avoid thinking in terms of “better/worse”) because both are great choices and it would be difficult to make a “mistake” with either.
This is also a matter of preference and each of them have their own different “feel” and compression curve. This article may be worth reading as well as post #7 here which may help you imagine how each may feel. Having said that …many heavier people may prefer Dunlop in the support layers because it has a higher compression modulus (which means it gets firmer faster with deeper compression and is more “supportive” in the same ILD as Talalay). You could also compensate for this by choosing firmer Talalay however. In the comfort layers … Talalay usually comes in softer versions and feels softer and more 'springy" than Dunlop (again assuming the same ILD) so this would depend on the surface feel you would prefer. The most reliable way to choose would be to personally test both of them and choose your favorite but in the absence of personal testing then the differences between them that you know about and your discussions with each manufacturer would be the best way to choose between them.
Phoenix